1. Field of the Invention
The present invention relates to a cutter structure for a shield machine which advances by boring through the tunnel wall of an existing tunnel.
2. Description of the Related Art
The present inventors have cited the device shown in FIG. 8 as a conventional shield machine which advances by boring through a tunnel wall in the interior of an existing tunnel.
As is illustrated in the figure, this shield machine a is one in which a cylindrical advancing section ring c, which is mounted on the advance opening part of an existing tunnel b, and a cylindrical advancing seal e which is slidably fitted over a shield frame d, are connected, whereupon the shield machine a is propelled upward by a propelling jack g which is under a reaction force to an advancing stand f. Then, as shown in FIGS. 9 and 10, due to the rotation of a cutter h the shield machine a bores through an excavatable wall i which has an arc-shaped cross section and forms one part of the existing tunnel b, thereby advancing upward.
In the cutter structure of this shield machine a, the shape of the cutter h itself is flat, and the excavating surface, which is formed from bits j and mounted on the cutter h, is also flat. Hence, when the shield machine a bores through the excavatable wall with an arc-shaped cross section using the cutter h, the outer circumference portion of the cutter h precedes the central portion such that the excavatable wall i is cut into but the central portion remains intact.
Moreover, as the cutter h continues to excavate upward, the portion connecting the central portion to the existing tunnel b gradually becomes smaller. Hence, the central portion becomes prone to collapse due to light outside pressure (earth pressure) and may also collapse prior to cutting by the bits j, whereby lumps of earth may be taken into the cutter chamber inside the shield frame d. Lumps of earth taken into a cutter chamber cannot be discharged by an earth discharging device which is designed in order to discharge regular earth and sand, and therefore block the earth discharging device.